The 2025 volume of the Vienna Yearbook of Population Research highlights the role of population inequality in demographic research. Besides classical markers of heterogeneity in individual behavior, such as sex/gender, age, education, urban-rural residence and socio-economic status, other sources of inequality related to generational, environmental, and spatial factors are covered in the volume. Understanding population inequality is key for modeling population developments and projecting them into the future. Equally important is to understand how and why different types of inequality arise and evolve, and what policy challenges they impose for socio-economic development, welfare systems and social cohesion.